1. As far as I know there has been full integration of the interior crew cabin to the exterior of the casules of the Apollos and of Soyus.

2. No Apollo can be launched on top of Ariana V, Wostok or Energija and no Soyus can be launched on top of an Ariane V or any US rocket because of diameters, docking mechanisms, interfaces etc.

Is that right?

What about developing "adapters" and the like then to provide that each capsule could be launched on top of each rcoket? This is interesting under the aspects of increased involvement into joint projects - ISS for example - and cooperation.

What about giving up the current full integration pof the crew cabin with the exterior of the capsules? The crew cabin with life support system, radio and so on could be module with interfaces to a caspeule around it. The casule would have the engines and the heat shield for example. This éxterior capsule could have a kind of gate at one side which allows for moving the crew cabin into it and connecting the interfaces. Another possibility could be to provide an upper and a lower part and to move the crew cabin into one of the parts first and then add the other part.

How are the current crew cabins integrated to the exterior capsules? And why?

All russian capsule systems (and derivations of it) have jettisonable aerodynamic shrouds, only the service module has integrated fairing. Soyuz and Shenzhou systems have separate command and re-entry modules.

Sounds as if the reason is and was the volume but not technical or safety reasons.

So there are no technical and safety arguments agianst the development of an interior of a capsule that could be integrated into an exterior capsule that can be launched on top of a Ariane 5 as well as into an exterior capsule that can be launched on top of one of Boeing's, Lockheed's or Northrop Grumman's rockets.

It would be a way by which NASA could make ESA launch astronauts.

Is the reason the Russians abandon the idea for non-human payload? In this case the solution could be that in some situations NASA launches the non-human payload and ESA launches the astronauts whereas in other situations NASA launches the astronauts and ESA the payload - the idea would allow to handle shifts of the optimum. Perhaps the Russians and JAXA could be involved too. They all would have their own vehicle but would share the interior of the manned capsule.